The Hill obtained an early copy of the letter and a press release, which asks Clinton if she will stand with them against the oil-sands pipeline.

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The Center for Biological Diversity is one of the 30 conservation groups that signed on to the letter, which will officially be sent to Clinton on Wednesday.

"Secretary Clinton, will you stand with us against Keystone XL?" the groups ask.

"Given your longstanding advocacy for the environment and the importance of battling the climate crisis, your involvement would lend an important voice to the struggle against this dangerous pipeline and in favor of energy sources that don’t threaten future generations of Americans," the letter states.

The added pressure on Clinton to take a side is a taste of what the likely presidential hopeful has in store as 2016 draws nearer.

"There’s no doubt that Keystone XL is a turning point for President Obama and whoever takes his place in 2016,” Bill Snape, senior counsel with the Center for Biological Diversity, said in a press release provided to The Hill. “Secretary Clinton’s voice is badly needed to help steer this decision in the right direction.”

Other groups that signed on include 350.org, CREDO, Friends of the Earth and Greenpeace. Notably absent from the list of signatories is the Sierra Club and Natural Resources Defense Council (NRDC), among other big environmental groups.

NRDC federal communications Edwin Chen said the group was approached in April about the letter but wanted to remain focused on those who actually had a say in the decision-making process at the State Department.

Still, staying quiet on the contentious $5.4 billion project could hurt Clinton, even if President Obama makes a decision long before she runs for office.

"We’re at a critical moment. Please join us," the letter states at the end.

Coming out on the pipeline is risky for the possible contender. On one hand it would give her an edge to secure possible contributions from big players like billionaire climate activist Tom Steyer, but it could hurt her in the general election.

"Coming out strong against Keystone XL gives Hillary a chance to show the climate movement that she stands with us, and not the fossil fuel industry," May Boeve, executive director of 350.org, said in joint press release compiled by the groups and provided to The Hill.

The CEO of CREDO Mobile adds that if Clinton does not stand against Keystone XL, then "environmental voters will know that she cannot be counted on in the fight against global warming.”